LU-8998 pfl: PFL feature implementation

Merge remote-tracking branch 'origin/pfl'

Please check out the HLD of PFL here:
        http://wiki.lustre.org/PFL_Prototype_High_Level_Design

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ide007c7fffc17dc6a51500b537f19d2391387f9d

diff --cc lustre/include/lustre/lustre_user.h
Simple merge

diff --cc lustre/include/lustre/lustreapi.h
Simple merge

diff --cc lustre/include/obd.h
Simple merge

diff --cc lustre/llite/dir.c
Simple merge

diff --cc lustre/mdc/mdc_internal.h
Simple merge

diff --cc lustre/mdc/mdc_request.c
Simple merge

diff --cc lustre/utils/liblustreapi.c
Simple merge